The present invention relates to a method and apparatus for controlling an air-fuel ratio of mixture to be supplied to a combustion engine, and more particularly to the ones for controlling a fuel injection time with learning control in response to a sensed air-fuel ratio.
In a conventional air-fuel ratio control system for a combustion engine, a fuel injection system has been employed to electronically control a fuel injection time. The fuel injection time has been determined by operating conditions of the combustion engine such as an intake air quantity, rotational speed of the engine or the like and by an ineffective fuel injection time which is independent of the operating condition.
To compensate for manufacturing differences and secular changes in operating characteristics of the fuel injection system, it has been proposed in U.S. Pat. Nos. 4,345,561 and 4,348,727 assigned to the same assignee of this patent application, for example, to employ a learning control in response to an air-fuel ratio of mixture sensed in an exhaust of the combustion engine. In the learning control, a plurality of correction values stored in a memory in correspondence to the operating conditions of the combustion engine are renewed in response to the sensed air-fuel ratio so that, when the combustion engine is operated in the same condition as in the previous time, the correction value renewed in the previous time and indicative of the most desirable correction is used to correct, in anticipation, the fuel injection time determined in dependence on the operating conditions.
The learning control, however, has not been directed to the other items such as the ineffective fuel injection time of fuel injectors which, although independent of the operating conditions, have manufacturing differences and secular changes.
Therefore, the characteristic changes of the system have not been compensated satisfactorily, resulting in the unsatisfactory air-fuel ratio control system.